CIRCLE. 



499 



SfrcV. step which lej to the construction of the celebrated in- 

 > "~Y~~* * strumcnt, which is the more immediate subject of our pre- 

 sent enquiries. 



When Hadley's octant became known to the world, 

 Professor Mayer was among those who rightly apprecia- 

 ted its valuable properties, and by extending its arc to a 

 complete circle, for the purpose of reducing the errors 

 of division, he gave it the repeating property of his go- 

 niometer, which he had before perhaps only thought of. 

 The Professor was fully aware, that the common theo- 

 dolite, and other surveying instruments, were capable of 

 being used as repeaters ; and we can assign no other rea- 

 son lor his giving the preference to the undivided bars, 

 diagonal scale and compasses, than that scales must at 

 that time have been more accurately divided than circles. 

 It was about the year 1789, when Borda first caused 

 a repeating circle to be constructed for astronomical and 

 geodetic purposes ; and one of our motives for being so 

 particular in what is said before, was to shew exactly how 

 much is due to him. The former repeaters, it has been 

 een, gave but one result for two operations ; but by the 

 use of an additional telescope, applied as hereafter to be 

 described, he procured the means of obtaining double re- 

 sults, but this advantage is not purchased at a less ex- 

 pence than three distinct operations. The astronomers 

 of the continent of Europe, and particularly in France, 

 where the art of dividing, except of kingdoms, is almost 

 unknown, have been lavish of their praiees of this instru- 

 ment ; while in this country, where that art is practised 

 with equal success upon minuter objects, a prejudice 

 against it has too long continued. As a proof of this, we 

 believe that the late Mr Ramsden never made either a re- 

 flecting or a repeating circle. The prejudice is now, how- 

 ever fast wearing off, and this circle begins to rake that 

 rank among our instruments to which its merit entitles it. 

 This we think will appear on perusing the following de- 

 scription, and from the examples as communicated by 

 different observers. 



Repeating Plate CXLVII. is a perspective representation of the re- 

 circle, peating circle as made in London, and exhibits, for ought 

 Purr. we know, as perfect a construction of that instrument as 

 CXLVII. }, ?s y e t appeared. In our description of it we shall, as 

 we proceed, occasionally remark upon the difference be- 

 tween it and those made abroad, of which this professes 

 to be in many respects a copy. 



At bottom, a strong tripod with feet screws rests upon 

 a pedestal. Two of the screws bear upon the concave 

 tops of two plain brass studs, which are affixed to the 

 pedestal, but the other screw rests upon an apparatus, 

 whereby an exceeding slow motion is communicated to 

 the instrument. This is represented at a. A steel spin- 

 dle 15 inches long, a full inch in diameter at one end, 

 and somewhat more than half as much at the other, has 

 its thicker end inserted into the middle of the tripod, to 

 which it stands perpendicular. This spindle or axis is 

 worked, especially near the ends, with the most scrupu- 

 lous attention to truth. A strong hollow brass pillar is 

 nicely centered upun the extreme ends of the spindle, 

 round which it turns, with a free and steady motion, and 

 resting with its base upon the middle of the tripod, sus- 

 tains the whole weight of the incumbent instrument. 



The azimuth circle 12 inches in diameter, united by 

 6 radii and a circular centre piece to the pillar, moves 

 round with it near to the upper surface of the tripod, 

 but yet without touching it. This circle is graduated 

 by lines into parts of 1(X each, which are met, and subdi- 

 vided to 10", by a vernier, as at 6, attached'to each branch 

 of the tripod. All this is executed with the greatest 

 care, in order that an angle may be well ascertained by 



a single observation, n cases where extreme accuracy is 

 not wanted ; and as the business of repeating is a very 

 laborious task, and productive of a vast waste of time, 

 this substitute will be considered by most observers as a 

 very convenient relief. It may here be noticed, that, as 

 the position of the telescope is fully three inches from the 

 vertical axis, all angles, except in astronomy, and where 

 land objects are rtry remote, must be subjected to a con- 

 sequent correction, in proportion to the distance of the 

 objects from the station. 



The advantage of three readings over two for the 

 limb of a theodolite, the telescope of which will turn 

 quite round, is great, and was perhaps first noticed by 

 Mr Pond. In two reverse positions of the circle, made 

 by turning the telescope half round in azimuth, and also 

 half round its own axis, three verniers will give the posi- 

 tion of an object upon six different and distant portions 

 of the circle ; but where two opposite verniers are used, 

 no more than two places of the circle are employed, for 

 on reversion the verniers only exchange positions. A 

 screw apparatus, as at c, attached to one of the branches 

 of the tripod, clamps the azimuth circle, or allows it 

 a quick or slow motion at pleasure. 



The repeating circle of Borda has an azimuth circle 

 of about four inches diameter, the single vernier of which 

 subdivides it to 3' or 5' : the edge is indented, and acted 

 on by a pinion, for the purpose of turning it round, in 

 the same manner that similar means are employed in the 

 common theodolite. 



To the top of the pillar is horizontally fised a strong 

 brass bar rf, upon which a similar bar is laid, and secu- 

 red there by two finger screws, the head of one of which 

 is represented at e : this is the part where the instru- 

 ment may be taken asunder for packing, or for more 

 conveniently moving it from place to place. At right 

 angles to the bars, and fastened to the upper one, are 

 erected two supporters, in the tops of which are cen- 

 tered the ends of a horizontal axis, round which the 

 whole of the upper part of the instrument may be turn- 

 ed. A semicircley is fastened at its centre, to one end 

 of this axis, and having its position paraliel and con- 

 tiguous to the interior side of one of the supporters, is 

 occasionally laid hold of by a clamp attached to the lat- 

 ter, and thus affords the means of securing the principal 

 circle and telescopes in any position that observation* 

 can require. 



The centre-work of the upper circle is made fast ta 

 the middle of the horizontal axis, which it crosses at 

 right angles ; at its remote end, upon the socket of the 

 centre, is placed a cylindrical counterpoise, which balan- 

 cing the circle and telescopes, keeps them, respecting 

 motion round the cross axis, at rest in every position. 

 The counterpoise, in order to give it a maximum ef- 

 fect, is fixed at the greatest distance from the cross 

 axis, which will allow it to pass the bar on the top of 

 the pillar, when the face of the circle is put horizontal, 

 or in changing its position from one side of the pillar to 

 the other. 



The principal circle is 18 inches in diameter, compo- 

 sed of an octagonal centre piece, into which are inserted 

 eight conical radii ; these being strong and tubular, are 

 united at their extremities by a circular limb ; strength- 

 ened in its place by a rim or hoop, which being screw- 

 ed fast to its back, forms what is termed an cage bar : 

 the division of the circle is by lines, the distance between 

 which is 10'. 



To the back of the octagonal centre piece it screwed 

 the axis whereon the circle and telescopes are turned 

 round in the plane of the circle : it is nearly eight inches 



Circle. 



